With increased focus on vehicle economy, automotive manufacturers are turning to smaller, lighter vehicles and unique vehicle powertrains to boost efficiency. Recirculated exhaust gas (“EGR”) is utilized in most conventional internal combustion engines to assist in the reduction of throttling losses at low loads, and to improve knock tolerance and reduce the level of oxides of nitrogen (“NOx”) in the exhaust gas. EGR is especially important as an emissions reducer in internal combustion engines that run lean of stoichiometry and are, as such, prone to emitting higher levels of NOx emissions.
One proposition that has been considered in the construction of internal combustion engine systems is to utilize one of a plurality of cylinders as a dedicated EGR source. Specifically, in a four cylinder engine for instance, three of the four cylinders will run at normal air, fuel and EGR mixtures. The exhaust gas produced by these cylinders will exit the internal combustion engine as exhaust gas and be treated in an exhaust gas treatment system prior to its release to the atmosphere. One of the four cylinders is operated at customized levels of air and fuel; as may be determined by an engine controller that is in signal communication with various engine, vehicle and exhaust system sensors. The exhaust gas produced in the single cylinder is transferred to the intake ports of the other three cylinders to provide EGR. Such a configuration allows, for example, richer EGR, which contains higher levels of Hydrogen, thereby improving knock resistance, fuel consumption and combustion stability while still allowing stoichiometrically combusted exhaust gas to be maintained in the exhaust gas treatment system for compatibility with the catalytic treatment devices.
A disadvantage to this type of internal combustion engine system is that a 4 cylinder internal combustion engine that uses only one cylinder as a dedicated EGR cylinder may not deliver uniform volumes or EGR to the working cylinders due to the uneven firing order of the cylinders. For example, the cylinder combustion event following the dedicated EGR cylinder combustion event is prone to receive more EGR diluent than the subsequent two firing cylinders. This variation in cylinder makeup (i.e. combustion air, fuel and EGR diluent) may result in uneven combustion performance that is difficult to control over a broad range of operating conditions.